This invention relates to improved thermoplastic compostions based on a vinyl aromatic compound-acrylo-nitrile copolymer and nitrile rubber. These compositions possess improved abrasion resistance and improved resistance to permanent deformation when subjected to tension or compression.
Copolymers of a 1,3-diene and acrylonitrile (generally referred to as nitrile rubber or NBR) are amorphous rubbers characterized by good oil, solvent and chemical resistance. Nitrile rubbers based on 1,3-butadiene and acrylonitrile are manufactured by the process of emulsion polymerization and are available commercially with acrylonitrile contents ranging from 20 to 50%. Nitrile rubbers generally exhibit poor mechanical properties until properly vulcanized, which is readily accomplished with vulcanizing agents commonly used in rubber processing including elemental sulfur, sulfur donors and organic peroxides. Nitrile rubbers are thus generally cross-linked during fabrication and are not normally reprocessible using thermoplastic equipment and methods of fabrication.
Copolymers of a vinyl aromatic compound and acrylonitrile such as, for example, styrene-acrylonitrile copolymers (SAN), are amorphous, rigid, thermoplastic materials. Molding compositions comprising SAN blended with NBR have also long been known for use as impact modified thermoplastics. Tough, thermoplastic compostions resistant to boiling water may be produced by melt mixing SAN and NBR in the presence of sulfur or other curing agents. The softening point of these blends is increased by curing the rubber component during blending to a soft vulcanized state, as described in U.S. Pat. No. 2,439,202. Processing difficulties can be overcome by increasing the toluene insoluble gel of the NBR to 30-50% prior to blending with the SAN resin, according to U.S. Pat. No. 2,550,139.
Most of the prior art SAN-NBR blends comprise lower levels of NBR, and are generally tough, impact resistant polymers lacking the resilience and elastic characteristics of rubber. SAN/NBR blends with increased rubber content provide extruded or injection molded products with poor tear strength and low elongation due to gross phase separation during processing. Grafting of SAN on NBR improves interfacial adhesion and reduces phase segregation during extrusion or injection molding. However, at high rubber contents (SAN/NBR ratios of 0.5/1, or less), the grafts exhibit objectionable melt elasticity and mold shrinkage; that is, the molded objects do not retain the shape of the mold but undergo substantial distortion. These graft copolymers are thus generally unsuitable for use as thermoplastic elastomers.
Thermoplastic compositions having elastomeric properties were more recently disclosed in U.S. Pat. No. 4,226,953. These elastomers, which comprise blends of SAN and nitrile rubber of high gel content (80% or more) are prepared by masticating the blend to advance the gel content of the NBR and form small particles of cured rubber dispersed in the thermoplastic SAN phase. This method suffers the disadvantage that the properties of the resulting products may vary significantly depending upon mixing time and temperature, shear rate, degree of dispersion and size of the rubber particles.
A truly thermoplastic molding composition comprising SAN and NBR which is readily processible by extrusion or injection molding and combines solvent resistance with good resilience and abrasion resistance would thus be a useful advance in the art.